The aim of this study is to get a better understanding of the variation of tritium concentration for monthly precipitation collected at Okinawa Island, the southwestern part of Japan. Tritium concentrations of monthly precipitation in Okinawa from June 2014 to December 2019 ranged from 0.05 to 0.27 Bq L^–1. Seasonal patterns of tritium concentration are determined with higher in spring and lower in autumn. The results of δD and δ¹⁸O indicate that the tritium concentration in winter is affected by the dilution effect of isotopes over the East China Sea. The stable isotope ratios and major ion components of tritium concentrations in monthly precipitation on Okinawa Island suggest that tritium concentrations are strongly influenced by the surrounding ocean throughout the year due to the dynamics of air masses. This study makes a significant contribution to the understanding of the behavior of tritium concentrations in monthly precipitation under the special environment of subtropical island regions. In order to investigate the source of high tritium concentration in monthly precipitation in the future, it is necessary to understand the supply of tritium from the stratosphere to the troposphere due to vertical convection of the atmosphere.

Strontium-90 (⁹⁰Sr) is an important radionuclide for understanding the effects of nuclear-related facilities on the surrounding environment, however the conventional method for determination of ⁹⁰Sr is complicated and time consuming. Therefore, we examined a rapid determination method for separation and purification of Y from agricultural and livestock products, indicator plants by means of solid-phase extraction with DGA resin. The resin is then used to determine ⁹⁰Sr by detection of yttrium-90 (⁹⁰Y), which exists in secular equilibrium with ⁹⁰Sr. The method was tested on ⁹⁰Sr-spiked samples (milk) and a certified reference material (IAEA-330), and the trueness values were 108% and 107% for milk, 111% for IAEA-330. In addition, stable Y recoveries exceeded 0.81 in the tests. The ratio of values measured by our examined method and the conventional method ranged from 0.75 to 1.15, and the ⁹⁰Sr detection limit ranged from 0.02 to 0.05 Bq/kg fresh weight. The sample-throughput time for the examined method was approximately 3 weeks shorter than that of the conventional method.

For the safety assessment of radioactive waste disposal, many environmental transfer parameters are used in biospheric dose assessment models. To set these parameter values, we generally refer to the database compiled by various organizations such as IAEA, NCRP, and USDOE, etc. Because transparency of the values is an important factor to provide reliable dose assessment results, the task group on parameters used in the models for radioactive waste disposal (FY 2019–2020) was aimed to discuss the current status of these parameters and to clarify future works necessary for dose assessment, especially in Japan. In this group, the following activities were carried out.

1) Trace-back the references of parameter values used in biospheric dose assessment for radioactive waste disposal in the past in Japan to confirm the original data.

2) Survey of newly reported parameter values in Japan and other countries.

3) Discuss on uncertainties of the parameter values and factors that affected the values.

4) Discuss reasonable methodologies to select more appropriate parameter values for specific dose assessments.

We provided herewith the summary report of this task group.

The probability of causation (PC) is known as a measure showing how much a disease after radiation exposure is attributable to the preceding exposure. PC is defined as a ratio of risk due to radiation exposure to a sum of baseline risk and the risk due to radiation exposure. For example, PC is calculated to be 50% where risk due to radiation exposure equals to the baseline risk, namely, in the situation where excess relative risk is 1. PC is a similar measure with those used in traditional epidemiology such as etiologic fraction, attributable fraction, attributable risk, etc. Recently, the term “assigned share” is more preferably used instead of PC mainly because PC is not exactly the probability, and because it is applied to the population not an individual. The basis for calculating PC is a cancer risk model developed by using epidemiological data of atomic bomb survivors in Hiroshima and Nagasaki. For this reason, the calculated PC values are dependent on extension of the follow-up period of A-bomb survivors, updates of cancer risk models including age and time modifying factors. Some of calculated PC values for selected cancer sites are shown here based on risk models derived from recent publications on atomic bomb survivors.

Clear radiation protection training and explanation tools for medical staff, patients, and caregivers during radiography are required. In this study, we developed visualization tool of scattered radiation distribution image of each X-ray room conditions. The 400 cases of X-ray room conditions were simulated using a Monte Carlo simulation code, Particle and Heavy Ion Transport code System (PHITS). The simulation conditions were (i) width and depth of the X-ray room (200 cm, 250 cm, 300 cm, 400 cm and 500 cm), (ii) opening or closing of the sliding door, (iii) direction of the X-ray table, (iv) X-ray posture, and (v) with or without protective clothing/shield. The scattered radiation distribution images were published on the Internet. The images are easily accessible on the Internet with selecting each radiography room condition. In radiation protection education, our tool can be used to (a) reduce radiation dose by keeping away from X-ray sources and scattered radiation sources, (b) shield scattered radiation by protective clothing and protective screens, and (c) prevent leakage of scattered radiation outside the room by closing the door of the radiography room. Our tool would improve the efficiency of radiation protection training for medical staff and alerting to patients and caregivers.